Broadband backward wave amplifier



Dec. 217? 11957 u J. F. HULL- BROADBAND BACKWARD WAVE AMPLIFIER 2 Sheets-Shes?. 1

Filed May 3l, 1955 FIGLB fmmmmm V/ ////1\HEATER SPACE CHARGE POWER OUT FIGA INVENTOR.

JOSEPH E HULL BY ZM WM A TTOR/VEY Dec. i?, N5? J. F. HULL. g'@

BRQADBAND BACKWARD WAVE AMPLIFIER Filed May 3l, 1955 2 Sheets-Sheet 2 POWER OUT 30 E 34, C9 POWER IN DUMMY NODE Q FIG. 6 v

POWER ouT INVENTOR. JOSEPH F. HULL ATT/P/VEY 2,817,040 BRoADBANn nAcKwAnD WAVE AMPLIFIER Application May 31, 1955, Serial No. 512,349 11 Claims. (Cl. S15-3.5)

(Granted under Title 35, U. S. Code (1952), sec. 266) assigner to the the Sec- The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment of any royalty thereon.

This invention relates to microwave amplifiers and more particularly to broadband backward wave magnetron type amplifiers employing multivelocity beams.

It is well known that amplification in traveling-wave type tubes is achieved by the interaction between a propagated electromagnetic wave and an electron beam moving at substantially the same velocity. An essential part of such traveling-wave tubes is the slow-wave structure adapted to propagate an electromagnetic wave slow enough so that the electron beam can be made to travel at the same speed. Loading a smooth waveguide periodically with lumped elements to form a perturbed transmission line will lower the phase velocity of the otherwise smooth transmission line. transmission line ideally suited for use as the slow-wave propagating circuit in magnetron type amplifiers is the interdigital waveguide structure which provides a relatively wide frequency coverage but with a rather limited gain. However, it has been found that when the anode voltage or beam current of such a structure is raised to provide increased gain over a prescribed bandwidth, oscillation occurs at a low cutol frequency which obviously limits the range of usefulness of such a structure as an amplier. This is `due to the fact that as the beam current is increased, backward oscillations will occur. It is well known that a Wave propagating energy in a direction opposite to the direction of electron ow produces eld components which have phase velocities in the same direction as the electron flow; viz, the negatively traveling space harmonics. If one of these components has a velocity essentially equal to that of the electron beam, that component will tend to modulate the stream to form bunches of electrons which will feed energy into the wave. Energy going into the circuit presumably will flow both ways but, at the frequency of operation, only the energy propagating back toward the source of the electrons can interact further. Hence the field acting on the electrons near the beginning of the tube is enhanced by the modulation of the beam and this eld produces more modulation. Consequently a tube of this type constitutes a continuous or distributed feedback system, so that if the beam current is increased, oscillation will occur at a frequency such that the phase and velocity conditions are correct.

It is therefore an object of the present invention to provide an improved magnetron type amplifier in which such limitations are overcome.

lt is another object of the present invention to provide an improved magnetron amplifier adapted to operate over a relatively broad band of frequencies with a relatively high gain.

It is still another object of the invention to provide a magnetron amplier operating as a backward wave amplilier and utilizing a multivelocity electron beam.

One type of perturbed 2,817,040 Patented Dee. 17, 1957 In accordance with the present invention there is provided a backward traveling wave amplifier wherein amplilication is achieved by the interaction of electromagnetic wave energy propagated in a direction opposite to the direction of travel of an electron beam. The propagating circuit comprises a perturbed waveguide structure having a dispersion characteristic such that it is adapted to pass a prescribed frequency range between Idiscrete upper and lower cutoff frequencies. Also included are means for generating a space charge comprising multivelocity electron beams and means for directing the path of the space charge in a direction opposite to the direction of propagation of the electromagnetic wave energy whereby the space charge is in coupling relationship to the electromagnetic wave energy. Included fur ther are means connected to the space charge generating means for limiting said beam velocities such that interaction with said propagated wave occurs only for frequencies above the lower cutoff frequency.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings in which:

Figure 1 illustrates a perturbed waveguide structure of the interdigital type;

Figure 2 illustrates the forward and backward wave dispersion characteristic of the line shown in Figure 1;

Figure 3 is an explanatory diagram illustrating the operation of a multivelocity beam in connection with the structure Iof Figure l; l

Figure 4 is a curve showing only the backward wave dispersion characteristic of the structure of Figure l;

Figure 5 shows one embodiment of the invention;

Figure 6 shows another embodiment of the invention, and

Figure 7 is a sectional view of the electron gun shown in Figure 6.

Figure 1 illustrates a rectangular waveguide adapted to propagate in the TELO mode which is modified by an interdigital structure placed longitudinally in the plane of the maximum E vector. Such structures are well known and provide suitable slow-wave propagating cir'- cuits for use in conventional traveling tubes and magnetron amplifiers of the type described in copending Kumpfer application Serial No. 170,875, tiled June 28, 1950, now Patent No. 2,760,111. The electromagnetic iield in such a periodically loaded guide can be expressed as a series of Waves having prescribed velocities which are known as space harmonics. It is well known that in order for an electron to move in a field such that the electron is caused to continually give up energy to the propagated Wave, the velocity of the electron must be such that it obeys the following equation:

a direction oppow/ Ve vs. f (w=21rf) where the expression w/Ve is known i as the propagation phase function or phase constant and is usually designated by ,89. Such a curve is illustrated in Figure 2 with the solid curve corresponding to dispersion characteristic of the 'forward space harmonics and the dotted curve corresponding to the dispersion*characteristic of backward space harmonics. To achieve synchronization, the electromagnetic wave may be considered to be divided into propagated modal components whose phase velocities correspond to Ve. VThe slope of a 'line such as OCD drawn through the origin represents a constant electron velocity over a wide frequency band and a line through the origin with a steeper slope would correspond t a higher electron velocity. If the traveling-wave tube incorporating the structure having the characteristics shown in Figure 2 were used as afor-ward wave ampliiier with the electron velocity correspondingv to the slope of the line OD, then it is apparent that the backward wave would interfere with the forward wave Vamplification and, as hereinabove described, oscillations would occur at a frequency corresponding to C if the beam current were great enough. Now, if only the backward characteristic curve were to be utilized by propagating an electromagnetic wave in a direction opposite to that of the electron beam, then such interference would be eliminated and some amplilication may be achieved but only .over a Vrelatively narrow bandwidth for any particular beam voltage. One method for increasing the bandwidth of such a backward wave amplitier is .to provide a multivelocity electron beam to interact with the fields established on the interdigital transmission line by the backward traveling wave. Such a system is illustrated in Figure 3 where there is shown an interdigital waveguide structure having the backward dispersion characteristics shown in Figure 4, and a linear cathode 12 positioned between one end wall of the waveguide and the interdigital transmission line 14. Although not shown, any suitable means may be provided for heating the cathode and supporting it in position within the waveguide. A magnetic field indicated at B normal to the plane Vof lthe paper is provided and a direct-current tield is applied inthe X direction by a suitable potential source V with the interdigital transmission .line 14 positive with respect to cathode 12 as shown. Such a structure is described in the aforementioned application. To operate as .a backward wave amplifier the input electromagnetic wave is applied through suitable coupling means at the beam 4collection end of the tube yand .the power output is taken from .the other end by other suitable coupling vmeans (coupling means not shown). Cathode 12 will provide a multi-stream electron beam having a continuous distribution of velocities corresponding from zero to the maximum velocity of the outermost electron whose velocity in the Z. direction varies in accordance withathe distance in the X direction asmeasured from the cathode. Of course, there is a :maximum boundary of space charge K indicated by the dashed line which is determined by the. magnitude of anode potential V and the strength of the magnetic lield indicated at B. Within the space charge, the potential varies as the square of the distance in the X direction. In a tube utilizing the structure shown in Figure 3, any attempt to increase the gain by increasing the anode voltage was found to cause the tube to oscillate or sing lat the k'cutol frequency wco inasmuch as the line impedance is theoretically infinite at this frequency. l

In accordance with the present invention, means are provided for eliminating the electron Avelocities having a velocity in the vicinity of cutoff frequency om, represented by line OM in Figure 4 so that no oscillations may occur and backward wave amplification may then be achieved over a relatively wide range. It is to be understood of course that the range of frequency to be amplified is .to be limited between they high frequency Vcutoff e, and low frequency cutol we., which is shown .on Figure 4 as the range between w1 and m2. Referring now to Figure 5,

4 there is shown at 30 a waveguide structure including an interdigital propagating circuit ,32 longitudinally positioned between the top and bottom walls of waveguide 30. Spaced from interdigital transmission line 32 along the same longitudinal plane thereof is an unperturbed (smooth) auxiliary or dummy lanode 34. Between one end wall of waveguide 30 and auxiliary anode 34 there is provided a linear cathode 36 which is coextensive only with anode 34. As shown, cathode 36 is opposite anode 34 .and parallel thereto. Both the auxiliary anode 34 and interdigital line 32 are maintained at a direct-current potentail with respect to cathode 36 by any suitable potential source with the negative terminal -HV applied to cathode 36, and anode 34 and interdigital line 32 being grounded as shown. Longitudinally spaced from linear cathode 36 and positioned between said one end wall of waveguide 30 and interdigital line 32 is a shaver electrode 38, which may be of any suitable cross-section, having a wedge-shaped portion 40 facing linear cathode 36 and `coextensive with line 32. Shaver electrode 38 is opposite interdigital line 32 and parallel thereto. A source of direct-current potential Vsis applied between cathode 36 and shaver anode 38 with the shaver anode y'arranged to be `positive with respect to cathode 36'as shown. A magnetic lield B is provided in a direction normal to the plane of the paper. `Due to the direct-current potential between auxiliary electrode 34 and linear cathode 30, and the magnetic eld B, there will be produced a multivelocity electron beam having a continuous ldistribution of velocity from zero to a maximum velocity ofthe outermost electron whose velocity in the Z direction varies according to how far from the cathode in the X direction one samples the beam. Conventional coupling means (not shown) are provided for coupling an electromagnetic wave into and out of the interdigital r transmission line in a direction opposite to Z. With the shaver electrode 38 having the transverse dimension S within the space charge from linear cathode 36, then the potential Vs is so chosen that shaver electrode 38 assumes the potential existing within the space charge at the distance S which lis a distance within the space charge corresponding substantially to the velocity designated by line OM in Figure 4. By such an arrangement, all electrons having a velocity below that represented by the distance S will be collected by shaver electrode 38 and thus be effectively removed from the interaction space between shaver 38 and line 32. All electrons above S will continue as though shaver electrode 38 were not present and the full space charge was in the volume occupied by shaver electrode 38. It is to be noted that the line impedance at w1 is lower than at wz. However, the electrons synchronizing at w1 are closer to the anode, and hence couple more strongly to interdigital line 32 than those at wz. Since these two effects tend to cancel each other, the overall gain over the bandwidth should be relatively constant.

Figure 6 illustrates another embodiment ofthe present invention. In Figure 6, the interdigital transmission `line 50 is provided 'with a longitudinal bore S2. To obtain a multivel'ocity beam in the direction Z, there is provided a multivelocity `'Pierce 'type electron gun 54 shaped in the'form of a parabola and having spaced concentric cathode emitting surfaces 56 which are appropriately insulated from each other. The interdigital line 50 is grounded .as shown and electromagnetic wave Venergy is applied thereto in a direction opposite to Z. Discrete negative potentials are respectively applied to `each of the concentric electron emitting surfaces such that the outermost emission surface is at the highest ynegative potential, designated as -I-IV, and succeeding rings are at successively smaller negative potentials designated as VA VB VC. Each of these negative potentials may correspond to one of the velocities indicated by respective straight lines through the origin of the curve shown in Figure '4 between w1 and o2. it is Vapparent that by selecting the appropriate potentials for VA, VB and VC etc., the velocity represented by the line OM may readily beeliminated. Transmission line 50 may be provided with a longitudinal magnetic focusing field coextensive with the transmission line.

In place of the parabolically-shaped cathode, cathode emitters in the form of a screen may be used with respective emitters being at different potentials VA VB VC etc. The beam which emerges into the delay line is obviously a multivelocity type beam.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in tre art that various changes and modilications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modiiications as fall within the true spirit and scope of the invention.

What is claimed is:

l. In a backward traveling-wave amplifier wherein amplification is achieved by the interaction of electromagnetic wave energy propagated in a prescribed direction at a prescribed velocity with an electron beam traveling in a direction opposite to said prescribed direction, means for generating a space charge comprising multivelocity electron beams, means for directing the path of the space charge in a direction opposite to the direction of propagation of said electromagnetic wave energy whereby said space charge is in coupling relationship with said wave energy, and means operatively associated with said electron space charge generating means for removing from said coupling space charge only those electrons having a velocity lower than a prescribed value.

2. In a backward traveling-wave 1amplifier wherein amplication is achieved by the interaction of electromagnetic wave energy propagated in a prescribed direction at a precribed velocity with an electron beam traveling in a direction opposite to said prescribed direction, a perturbed waveguide structure for propagating said electromagentic wave energy and having a dispersion characteristic such that it is adapted to pass a prescribed frequency range between discrete upper and lower cutoff frequencies, means for generating a space charge comprising multivelocity electron beams, means for directing the path of said space charge in a direction opposite to the direction of propagation of said electromagnetic wave energy whereby said space charge is in coupling relationship to said electromagnetic wave energy, and means connected to said electron space charge generating means for limiting the beam velocities in said space charge such that interaction with said propagated wave occurs only for frequencies above said lower cutoif frequency.

3. The backward traveling-wave y.amplifier in accordance with claim 2 wherein said last mentioned means comprises an electrode within said space charge opposite said perturbed waveguide structure and parallel thereto, and means for maintaining said electrode at -a prescribed positive potential with respect to said space charge gener- .ating means.

4. The backward traveling-wave amplifier in accordance with claim 2 wherein said multivelocity electron beam generating means comprises an electron gun in the form of a parabola having spaced electron emitting concentric rings, insulated from each other, and the means for limiting the beam velocities comprises discrete sources of negative potentials respectively connected to each of said rings such that the outermost ring is at the highest negative potential and respective succeeding rings are at successively smaller negative potentials.

5. The backward traveling-wave amplifier in accordance with claim 4 wherein said perturbed waveguide structure is an interdigital transmission line.

6. In a backward traveling-wave amplier wherein ampliication is achieved by the interaction of electromagnetic wave energy propagated in a prescribed direction with an electron beam traveling in a direction opposite to said prescribed direction, a perturbed waveguide for propagating said electromagnetic wave energy and having a dispersion characteristic such that it is adapted to pass a prescribed frequency range between discrete upper and lower cutoff frequencies, an auxiliary anode longitudinally aligned with said perturbed waveguide and spaced therefrom, means positioned opposite said auxiliary anode and spaced therefrom for generating a space charge comprising multivelocity electron beams, means for directing the path of said space charge in said opposite direction whereby said space charge is in coupling relationship with said wave energy, and means within said space charge positioned opposite said perturbed waveguide and spaced from said space charge generating means for removing from said coupling space charge all electrons having a velocity substantially equal to and lower than the velocity corresponding to said lower cutoif frequency.

7. The backward traveling-wave amplifier set forth in claim 6 wherein said last mentioned means and said perturbed waveguide are coextensive.

8. The backward traveling-wave amplifier set forth in claim 6 wherein said perturbed waveguide is an interdigital transmission line.

9. In a backward traveling-wave amplifier wherein ampliiication is achieved by the interaction of electromagnetic wave energy propagated in a prescribed direction with an electron beam traveling in a direction opposite to said prescribed direction, a perturbed waveguide structure for propagating said electromagnetic wave energy and having a dispersion characteristic such that it is adapted to pass a prescribed frequency range between discrete upper and lower cutoif frequencies, an auxiliary anode longitudinally aligned with said perturbed waveguide and spaced therefrom, a linear cathode parallel to said auxiliary anode and coeXtensive therewith, and adapted to emit a space charge comprising multivelocity electron beams, means for directing the path of said space charge in said opposite direction whereby said space charge is in `coupling relationship with said wave energy, a shaver electrode Within said space charge parallel to said perturbed waveguide structure and spaced from said linear cathode, and means for biasing said shaver electrode with respect to said cathode whereby all electrons having la velocity substantially equal to and lower than the velocity corresponding to said lower cutoff frequency are collected by said shaver electrode.

10. A broadband backward wave amplifier comprising, means for propagating electromagnetic wave energy in a prescribed direction at a prescribed velocity, means for generating a space charge comprising multivelocity electron beams, means for directing the path of said space charge in a direction opposite to the direction of propagation of said electromagnetic wave energy whereby said space charge is in coupling relationship with said wave energy, and means in circuit with the electron space charge generating means for limiting the beam velocities in the coupling space charge to values such that interaction with said propagated wave occurs only for frequencies higher than a prescribed cut-oif frequency.

11. The backward wave amplifier in accordance with claim 10 wherein said last mentioned means comprises an electrode within the coupling space charge, and means for biasing said electrode with respect to the electron space charge generating means whereby all electrons having a velocity substantially equal to and lower than the velocity corresponding to the prescribed cut-off frequency are collected by said electrode.

No references cited. 

